Just put together this contraption today... it'll go with me on backpacking trips where I need more than 60 hours of GPS time! No idea how many times it'll charge the watch, but I'm guessing at least 5 full charges (this battery pack is 3350mah).
$5 for the cable (aliexpress) and $10 for the charger (amazon). It can also give my phone or other USB devices a bit of juice, in a pinch. Haven't had a chance to weigh it yet, but it feels like about 5 ounces.